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An Ethernet hub or concentrator is a device for connecting multiple twisted pair or fibre optic Ethernet devices together, making them act as a single segment. It works at the physical layer of the OSI model, repeating the signal received at one port out each of the other ports (but not the original one). The device is thus a form of multiport repeater. Ethernet hubs are also responsible for forwarding a jam signal to all ports if it detects a collision.

Hubs also often come with a BNC and/or AUI connector to allow connection to legacy 10BASE2 or 10BASE5 network segments. The availability of low-priced Ethernet switches has largely rendered hubs obsolete but they are still seen in older installations and more specialist applications.

 


Technical information

A hubbed Ethernet network behaves like a shared-medium, that is only one device can successfully transmit at a time and each host remains responsible for collision detection and retransmission. With 10BASE-T and 100BASE-T links (which generally account for most or all of the ports on a hub) there are separate pairs for transmit and receive but they are used in half duplex mode in which they still effectively behave like shared medium links (See 10BASE-T for the pins specifications)

The need for hosts to be able to detect collisions limits the number of hubs and the total size of the network. For 10 Mbit/s networks, up to 5 segments (4 hubs) are allowed between any two end stations. For 100 Mbit/s networks, the limit is reduced to 3 segments (2 hubs) between any two end stations, and even that is only allowed if the hubs are of the low delay variety. Some hubs have special (and generally manufacturer specific) stack ports allowing them to be combined in a way that allows more hubs than simple chaining through Ethernet cables, but even so a large Fast Ethernet network is likely to require switches to avoid the chaining limits of hubs.

Most hubs detect typical problems, such as excessive collisions on individual ports, and partition the port, disconnecting it from the shared medium. Thus, hub-based Ethernet is generally more robust than coaxial cable-based Ethernet, where a misbehaving device can disable the entire segment. Even if not partitioned automatically, a hub makes troubleshooting easier because status lights can indicate the possible problem source or, as a last resort, devices can be disconnected from a hub one at a time much more easily than a coaxial cable. They also remove the need to troubleshoot faults on a huge cable with multiple taps.

Dual speed hubs

In the early days of Fast Ethernet, Fast Ethernet switches were relatively expensive devices. However, hubs suffered from the problem that as simple repeaters they could only support a single speed. Whilst normal PCs with expansion slots could be easily upgraded to Fast Ethernet with a new network card, computers with less common expansion mechanisms, or no expansion bus at all, and other equipment, such as printers, could be expensive or impossible to upgrade. Therefore, a compromise between a hub and a switch appeared known as a dual speed hub. These split the network into two segments, each acting like a hubbed network at its respective speed with a two-port switch between them. Therefore they allowed mixing of the two speeds without the cost of a Fast Ethernet switch.

Usefulness

Historically, the main reason for purchasing hubs rather than switches was price. This has largely been eliminated by reductions in the price of switches, but hubs can still be useful in special circumstances:

  • A protocol analyzer connected to a switch does not always receive all the desired packets since the switch separates the ports into different segments. Connecting the protocol analyzer to a hub allows it to see all the traffic on the segment. (Expensive switches can be configured to allow one port to listen in on traffic on another port. However, these cost much more than a hub.)
  • Some computer clusters require each member computer to receive all of the traffic going to the cluster. A hub will do this naturally; using a switch requires implementing special tricks.
  • When a switch is accessible for end users to make connections, for example, in a conference room, an inexperienced or careless user (or saboteur) can bring down the network by connecting two ports together, causing a loop. This can be prevented by using a hub, where a loop will break other users on the hub, but not the rest of the network. (It can also be prevented by buying switches that can detect and deal with loops, for example by implementing the Spanning Tree Protocol.)
  • A cheap hub with a 10BASE2 port is probably the cheapest and easiest way to connect devices that only support 10BASE2 to a modern network (cheap switches don't tend to come with 10BASE2 ports). The same goes for linking in an old thicknet network segment using an AUI port on a hub (individual devices that were intended for thicknet can be linked to modern Ethernet by using an AUI-10BASE-T transceiver).

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